For basic duplexing schemes adopted in a cellular system, as illustrated in FIG. 1, a Time Division Duplexing (TDD) mode refers to transmission of uplink-downlink signals in the uplink and the downlink in different periods of time over the same operating frequency band, where there is a Guard Period (GP) between the uplink and the downlink; and a Frequency Division Duplexing (FDD) mode refers to possible transmission of uplink-downlink signals in the uplink and the downlink concurrently over different frequency carriers in different operating frequency bands, where there is a Guard Band (GB) between the uplink and the downlink.
A frame structure of a Time Division-Long Term Evolution (TD-LTE) system is somewhat complicated, as illustrated in FIG. 2, where a radio frame with a length of 10 ms includes 10 subframes in total including special subframes and normal subframes, and each subframe is 1 ms in length. The special subframe includes three time slots: a Downlink Pilot Time Slot (DwPTS) for transmitting a Primary Synchronization Signal (PSS)/Physical Downlink Control Channel (PDCCH)/Physical HARQ Indication Channel (PHICH)/Physical Control Format Indication Channel (PCFICH)/Physical Downlink Shared Channel (PDSCH), etc., a GP for a guard period between the downlink and the uplink, and an Uplink Pilot Time Slot (UpPTS) for transmitting a Sounding Reference Signal (SRS)/Physical Random Access Channel (PRACH), etc. The normal subframes include uplink subframes and downlink subframes for transmitting uplink/downlink control signaling, traffic data, etc.
A radio frame can be configured with two special subframes (in the subframes #1 and #6) or can be configured with one special subframe (in the subframe #1). The subframe #0 and the subframe #5, and the DwPTS time slot(s) in the special subframe(s) are usually used for downlink transmission, the subframe #2 and the UpPTS time slot(s) in the special subframe(s) are usually used for uplink transmission, and the remaining subframes can be configured for uplink transmission or downlink transmission as needed.
In the TD-LTE system, the total length of three time slots DwPTS/GP/UpPTS in a special subframe is 1 ms, as depicted in Table 1, where different configuration conditions for the allocated lengths of these three time slots are supported, and there is a temporal length unit of Ts with Ts=1/(15000×2048) second.
TABLE 1Configuration format of TD-LTE special subframesShort CPExtended CPConfiguration No.DwPTSGPUpPTSDwPTSGPUpPTS0 6592 · Ts21936 · Ts 2192 · Ts 7680 · Ts20480 · Ts 2560 · Ts119760 · Ts8768 · Ts20480 · Ts7680 · Ts221952 · Ts6576 · Ts23040 · Ts5120 · Ts324144 · Ts4384 · Ts25600 · Ts2560 · Ts426336 · Ts2192 · Ts 7680 · Ts17920 · Ts 5120 · Ts5 6592 · Ts19744 · Ts 4384 · Ts20480 · Ts5120 · Ts619760 · Ts6576 · Ts23040 · Ts2560 · Ts721952 · Ts4384 · Ts———824144 · Ts2192 · Ts———
Seven different allocation schemes of uplink-downlink subframes are supported in the TD-LTE system with their particular configuration parameters as depicted in Table 2, where D represents downlink transmission, U represents uplink transmission, and S represents a special subframe including three time slots DwPTS, GP and UpPTS.
TABLE 2LTE TDD uplink-downlink subframe configurationsConfigur-ationSwitchSubframe numberNumberperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The special subframe configuration and the uplink-downlink subframe allocation scheme above are broadcasted to all of User Equipments (UEs) in a cell by a base station in System Information (SI).
As can be apparent from the foregoing description, a UE can be instructed by modifying system information to change the two frame configuration parameters above in the TD-LTE standard. The shortest frame configuration modification periodicity of 640 ms supported in the TD-LTE standard has failed to fully accommodate a dynamically variable demand for traffic.
A dynamic solution to allocation of uplink-downlink subframes has been proposed in the prior art as follows:
In a specific period of time, there are four preset types of subframes including subframes fixed for downlink transmission, subframes fixed for uplink transmission, and subframes flexibly allocated for uplink or downlink transmission. As illustrated in FIG. 3, for example, the period of time is one radio frame (which is merely illustrative although an alternative period of time may be also possible), where the subframes #0 and #5 are fixed downlink subframes, the subframes #2 and #7 are fixed uplink subframes, the subframes #1 and #6 are special subframes (or can be classified into fixed downlink subframes), and the remaining subframes (#3, #4, #8 and #9) are subframes allocated flexibly for uplink or downlink transmission. The last type of subframes can be configured dynamically by the base station in view of a real-time traffic demand and a real-time channel condition to accommodate a dynamic change in traffic demand.
A method of configuring dynamically TDD subframes has been proposed in the disclosed solution of the prior art. In a real system, if TDD subframes are configured dynamically so that different cells are preset with different uplink-downlink subframe configurations, then interference across time slots may occur with adjacent cells. The adjacent cells here may be geographically adjacent cells over the same TDD carrier, as illustrated in FIG. 4, and given the same TDD carrier, a downlink signal of a base station in a macro cell may interfere with an uplink signal of a UE in a femto cell, which may in turn interfere with the downlink signal in the macro cell. Alternatively, the adjacent cells can be geographically overlapping or adjacent cells over adjacent TDD carriers, as illustrated in FIG. 5. The adjacent cells here may be cells deployed at the same level. e.g., macro cells deployed at the same level, or can be cells deployed at different levels. e.g., a macro cell and a micro cell etc.
As can be apparent, the problem of interference across time slots accompanying the solution to dynamic configuration of TDD subframes proposed in the prior art has not been addressed yet, thus degrading the performance of the system; and taking a cell as an example, interference thereto in each uplink subframe may be uplink interference of a UE served by an adjacent base station or may be downlink interference of the adjacent base station. If the interference thereto arises from downlink interference from the adjacent base station at a short distance from a cell in question, then there may be strong interference of the adjacent base station to the cell in question, thus possibly resulting in significant interference to the uplink subframe in the cell in question and consequentially deteriorating the performance of the system. As a result, the performance of the system can not be guaranteed with the traditional dynamic configuration of TDD subframes.